<<How can you possibly have 4 levels in one class period?>>
This is the norm at the small, private school where I teach.... Fortunately though, my classes are SMALL so it's do-able.
But also, to make things practical, I have a somewhat set curriculum for Art I kids and for the "Art Others" I have to reinvent the wheel each year and come up with unique projects while still teaching certain concepts. Kids who have more ability (regardless of "level") work at higher end projects and often have more freedom to choose projects.
I'll admit that I LIKE having this type of freedom, since it's a private school, but it would be much easier if I could repeat more projects....